I think a better way to measure car efficiency is to do miles per energy consumed in Joules or KJ.
Efficiency, yes, but since we don't buy fuel by it's energy rating and the cost per KJ varies dramatically between fuel/energy sources, and because the energy density varies dramatically between fuels, etc., it's a useless number to the consumer.
Estimated cost per 1000 miles (per 1000 KM for most of the world) is far more useful to the buyer. This gives easy access to estimating monthly energy/fuel costs.
Then, you can add a vehicle pollution index that is based upon energy efficiency and the fuel/energy source. Pollution index should be calculated for the vehicle from the "pump"/charging outlet, perhaps in kJ/mi (kJ/km). You can add a separate pollution index for the fuel/energy source that estimates the pollution created during production & distribution of that fuel/energy source, expressed in "pollution units"/kJ. Multiply the pollution index of the fuel/energy at the point of distribution times the vehicle pollution index and you get "pollution units" per mi (or km).
The hardest part of that is agreeing on the "pollution units" because not all pollutants are equally harmful. Still, it would give easy access to estimating how "green" a given vehicle is using any of it's available energy/fuel sources. You could choose to buy the cheapest fuel/energy, or the least polluting source depending upon what's more important to you. For instance, an EV owner might choose to buy more expensive electricity from a 100% renewable source, or just buy what's cheapest knowing the efficiency of the EV and centralized electricity generation is less polluting than a gasoline powered vehicle. You might even change your priorities from time to time during the life of the vehicle.